We have measured the photoluminescence decay time of the P, R, and S bound excitons at the neutral nitrogen donors in 6H SiC using picosecond pulsed excitation. At 2 K the decay times are 8.0, 1.8, and 1.5 ns, respectively, which are significantly faster than previously reported values for shallow donors in other indirect-band-gap materials such as Si or GaP. Each of the observed decay times is found to be independent of the doping level in the sample, and also temperature independent at low temperatures, but decreases when the bound excitons are thermally ionized. The decay time related to different donors exhibits a strong dependence on the binding energy of the donor level. We suggest that the dominating mechanism responsible for the observed decay time is a phononless Auger process. In high-purity samples we have also measured the free-exciton decay time at low temperatures to be 12 ns.

In this paper two things are done. (1) A projection-operator formalism is used to derive the time-convolutionless stochastic equation of motion for the reduced density operator from the quantum Liouville equation for an arbitrary driven system coupled to a stochastic reservoir. As an initial condition, decoupling of the system and reservoir for the total-density operator is assumed in the formulation. Perturbation expansions of the generalized collision operator are carried out in powers of the driving field within the Born approximation for the interaction of the system with the reservoir. The time-convolutionless form of the equation for the reduced density operator allows one to include the memory effects systematically. (2) Time-convolutionless quantum kinetic equations for interacting electron-hole pairs near the band edge in semiconductors under an arbitrary optical field are obtained from the equation of motion for the reduced density operator. These equations generalize the semiconductor Bloch equations to incorporate the non-Markovian relaxation and the interference effects between the external driving field and the stochastic reservoir of the system and are valid to any time scale. It is shown that the interference term modulates the interband polarization and includes the renormalized memory effects.

A detailed light-induced electron-spin-resonance study of iron and chromium impurities in ZnSe is presented. The position of the Fe 2+/3+ energy level in the band gap of ZnSe is determined. The role of iron and chromium impurities in nonradiative recombination processes in ZnSe is discussed.

Hydrodynamical fluctuations of the electron gas comprise low-frequency and long-range stochastic excitations over a steady state of the system. In this work we develop the theory of hydrodynamic fluctuations in the nonequilibrium electron gas which can be described by its electron temperature. For this case the characteristic spatial-time parameters are the electric charge decay time ${\mathrm{\tau }}_{\mathit{M}}$, the electron temperature relaxation time ${\mathrm{\tau }}_{\mathit{T}}$, and two corresponding diffusion lengths ${\mathit{L}}_{\mathit{M}}$ and ${\mathit{L}}_{\mathit{T}}$, respectively. The spectral densities of the fluctuations are found and investigated for arbitrary relationships between the fluctuation frequency ω and times ${\mathrm{\tau }}_{\mathit{M}}$ and ${\mathrm{\tau }}_{\mathit{T}}$, for wave vectors q and lengths ${\mathit{L}}_{\mathit{M}}$, ${\mathit{L}}_{\mathit{T}}$. In the first case, we established the effect of the crossover correlation of the electron density δn(r,t) and temperature δT(r,t) fluctuations. The cross correlation depends on ω and changes its sign, which indicates the existence of the frequency range of correlation and anticorrelation of δn(r,t) and δT(r,t). In the general case, spectral densities of the fluctuations have non-Lorentz form, which also holds with thermal equilibrium. At the equilibrium the cross-correlation effect leads to only redistribution of the fluctuation intensity over the frequency region. Under nonequilibrium conditions the cross correlation also changes the integral intensities of the fluctuations and is directly related to additional kinetic correlation of the hot electrons caused by electron-electron interaction. The results are applied to the calculation of light scattering by electron plasma fluctuations. The cross-correlation effect gives the essential contribution to the cross section of the light scattering. It is shown that the additional kinetic correlation of hot electrons can be directly measured by means of the light-scattering experiment.

We present results of a first-principles molecular-dynamics study of structural, dynamical, and electronic properties of liquid Ge. In agreement with experiments, the electronic density of states shows that liquid Ge is metallic. However, an analysis of the electronic charge density, pair correlation function, and structure factor shows the existence of some covalent bonds in the liquid. These bonds give rise to broad bands in the power spectrum, reminiscent of the vibrational modes of crystalline Ge. They are also responsible for the low coordination number found in the melt. The calculated diffusion coefficient is also in agreement with available experimental results.

We measure the time- and energy-resolved luminescence due to hot biexcitons in CdSe at 8 K using picosecond upconversion spectroscopy. The biexciton luminescence rises with a time constant of 12 ps following picosecond photoexcitation of electrons and holes. It then decays with a time constant of 240 ps. The time dependent luminescence spectrum is used to deduce the hot biexciton energy relaxation rates in CdSe. We compare the results with theoretical calculations based on phonon emission by biexcitons.

The frequency distribution of the acoustic phonons emitted from a biased metal-insulator-metal normal-state tunnel junction has been studied. Electrons tunneling across the insulating layer form an excited population with a range of energies up to ∼${\mathit{eV}}_{\mathit{B}}$ above the Fermi level and then relax, producing a phonon spectrum with a frequency cutoff at the same characteristic value. Normal-state tunnel junctions made from aluminum have been produced and phonon spectra determined by using the stress-tuned splitting of boron acceptors in silicon as a spectrometer. It has been shown that, in the range ∼1–4 meV (∼250–1000 GHz), the second differential of the signal with respect to junction bias (${\mathrm{\partial }}^2$P/∂${\mathit{V}}_{\mathit{B}}^2$) is quasimonochromatic, having a spectrum of an approximately Gaussian form with the peak occurring at an energy of ${\mathit{eV}}_{\mathit{B}}$ and a half width of ∼6${\mathit{kT}}_{\mathit{e}}$.

A general multiband and multichannel scattering theory of the current in mesoscopic-device structures is developed and applied to the Zener diode. It takes into account the realistic electronic structure, its modification by the high electric field, and the field-free contact regions in a nonperturbative manner. This theory elucidates the interplay between Zener tunneling and Wannier-Stark resonances. Quantitative conditions for the occurrence of Wannier-Stark oscillations in the current of a bulk semiconductor or superlattice are derived. It is predicted that Wannier-Stark resonances are detectable in the interband tunneling current of highly doped submicrometer p-i-n diodes with very short i zones. We show that there are two regimes in the Zener tunneling current: a low-field or Zener regime where the conductance is a smooth function of the applied voltage, and a high-field or Stark regime where Wannier-Stark resonances are induced.

The anharmonicities of the C-H stretch modes in HCN and the passivated C acceptor in GaAs are investigated using ab initio local-density-functional cluster theory. The effective-mass parameter χ for the C-H stretch mode is shown to be less than unity in HCN, and greater than unity for the GaAs case. The calculated anharmonic parameter for the first defect is found to be 106 ${\mathrm{cm}}^{\mathrm{-}1}$ and is in very good agreement with experiment. For the second defect, the anharmonicity is about 50% larger in agreement with empirical estimates. The frequencies of the fundamental transitions in both systems are shown to be very sensitive to the C-H length. This limits the accuracy of theoretical investigations of these high frequency H modes. Finally, the effects of electrical anharmonicity are considered and it is shown that they reduce the intensity of the overtone in the C-H complex in GaAs by about 70%.

The BC8 structure (body-centered cubic with eight atoms per cell) is a known pressure-induced modification of both silicon and germanium. However, its diatomic analogue [the SC16 structure (a simple cubic lattice with a basis of 16 atoms)] has never been found in compound semiconductors. We find from total-energy pseudopotential calculations that the SC16 structure is a stable high-pressure polymorph of the III-V semiconductors GaAs, InAs, and AlSb. We report ab initio calculations of the structural, electronic, and vibrational properties of SC16-GaAs. The wurtzite structure is found to be unstable at all pressures for each compound considered. We consider possible transition routes consistent with our high-pressure x-ray diffraction results and propose that the formation of the SC16 structure in compounds is kinetically inhibited by the formation of wrong bonds at the structural transition.

The nucleation of diamond on silicon (100) in a methane-in-hydrogen microwave plasma has been investigated by atomic-force microscopy, scanning electron microscopy, and reflection high-energy electron diffraction (RHEED). The nucleation of diamond was performed by application of an electrical substrate potential. It was found that three-dimensional nonfacetted islands are initially formed whose sizes increase with the deposition time. In spite of their very small sizes of a few nanometers, RHEED reveals that the islands are of crystalline-diamond structure. An induction time of about 6.5 min was necessary for the diamond nucleation, which is partly caused by the formation of a silicon-carbide surface layer due to the carbon diffusion into the substrates. The time dependence of nucleation density was investigated and fitted with a model kinetics, which considers the formation, destruction, and capture of active sites, germs, and nuclei. Analyses of the first-nearest-neighbor distance distributions reveal the formation of a depletion zone of nuclei around the existing islands. These results confirm the role of the adatom diffusion improved by the bias-enhanced ion bombardment. Analyses of the island-size and the island-height distributions show constant growth rate at the beginning of the diamond deposition. From the nearly constant ratios of the island height to island size, independent of the nucleation time, one can deduce that the thermodynamics accounts not only for the growth mode, but also for the island shape during deposition. The bias-enhanced ion bombardment during deposition may also increase the diffusion of the surface atoms of the islands, allowing the system to approach equilibrium.

We outline a method to determine the intensity of the second-harmonic radiation emitted from a semiconductor surface. A calculation carried out for As/Si(111) allows us to discuss details of the nonlinear response such as the relative contributions of transitions resonating at the incident or at the emitted frequency. We also show that second-harmonic generation can be used as a tool for surface spectroscopy. We also show that the intensity of the emitted radiation, rather than the nonlinear susceptibility, should be compared with experiment, since the intensity is less sensitive to computational details than the second-order susceptibility.

We present a theory of phonon-assisted tunneling in GaAs/AlAs double-barrier structures, which treats the electrons in the emitter as a two-dimensional electron gas. A complete set of confined and interface optical-phonon modes is calculated using a dielectric-continuum model, and we derive a general expression for the electron-phonon Hamiltonian valid for all optical-phonon modes in an arbitrary heterostructure. The electronic wave functions relevant to phonon-assisted tunneling are found by self-consistently solving the Schrödinger and Poisson equations in both the well and the emitter. Five different phonon modes are predicted to dominate the phonon-assisted tunneling current: three LO-like interface modes, the half-space modes in the emitter, and the confined modes in the well.

The temperature-dependent (10–300 K) optical band gap ${\mathit{E}}_0$(T) of the epitaxial metastable zinc-blende-structure β-GaN(001)4×1 has been determined by modulated photoreflectance and used to interpret low-temperature photoluminescence spectra. ${\mathit{E}}_0$ in β-GaN was found to vary from 3.302±0.004 eV at 10 K to 3.231±0.008 eV at 300 K with a temperature dependence given by ${\mathit{E}}_0$(T) =3.302–6.697×${10}^{\mathrm{-}4}$${\mathit{T}}^2$/(T+600) eV. The spin-orbit splitting ${\mathrm{\Delta }}_0$ in the valence band was determined to be 17±1 meV. The oscillations in the photoreflectance spectra were very sharp with a broadening parameter Γ of only 10 meV at 10 K. The dominant transition observed in temperature-dependent photoluminescence was attributed to radiative recombination between a shallow donor, at ≃11 meV below the conduction-band edge and the valence band.

We investigate the temporal evolution of the instantaneous frequency of ultrashort-pulse four-wave mixing in GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$As quantum-well structures. We find that the coherent light emitted by excitons exhibits a complex phase behavior which depends critically on the density of excitons and the central frequency of the exciting pulses. Depending on the excitation conditions, we observe nonlinear shifts of the instantaneous frequency within one ultrashort pulsed emission or quantum beats. In the latter case we determine that the beat π-phase shift is very fast, only 40% above the fundamental quantum limit. We also present elaborate numerical simulations of the experiments based on a six-band generalization of the semiconductor Bloch equations. This formalism takes into account Coulomb many-body interactions and Pauli exclusion. It reproduces the salient features of the experimental data. It fails to account for important details revealed by the very sensitive phase measurements. The discrepancies can be traced back to two approximations in the theory: the statical treatment of screening, and the Lorentzian description of dephasing. In both cases, this indicates the need for theoretical refinements requiring a microscopic description of Coulomb scattering and dephasing processes in a non-Markovian theory.

We report a direct observation of surface reconstruction induced by the adsorption of ${\mathrm{C}}_{60}$ molecules. At submonolayer and full monolayer coverage of ${\mathrm{C}}_{60}$ on Ge(111), two surface phases (3 √3 ×3 √3 R30° and √13 × √13 R14°) as well as a localized metastable 5×5 phase are identified by low-energy electron diffraction and scanning tunneling microscopy. The formation of the two phases is facilitated by induced reconstructions of the Ge surface into different surface structures. Models for these surface structures are proposed that elucidate the close interplay between the adsorbed ${\mathrm{C}}_{60}$ molecules and the host Ge surface.

We have studied the effect of electron-electron interaction on the magnetic moment (associated with the persistent current) of electrons in a quantum ring. We have introduced a model where the electron makes a circular motion in a parabolic confinement simulating a quantum ring which is subjected to a perpendicular magnetic field. The electron states in such a ring with and without the Coulomb interaction are then investigated. We have also explored the limits of narrow and wide rings. Our detailed calculations of the energy spectrum where the interelectron interactions are also included indicate that when the electrons occupy the states of only the lowest Landau band, the interaction merely shifts the spectrum to higher energy and its effect on magnetization is insignificant. The result can be understood as entirely due to conservation of the angular momentum. A qualitative estimate for a large system where electrons also occupy higher Landau bands indicates that the Coulomb interaction mixes some of the states resulting in a rapid increase in the magnetization.

In this paper we present a scattering-matrix formalism to study electron transport in a mesoscopic system such as a lateral antidot array with a strong modulation potential. We show that the physically important and less localized states are allowed to dominate in the implementation of the formalism and, therefore, the problem of the numerical instability that one often encounters in the application of the transfer-matrix method to lateral electron transport has been solved. As an example of its application, the formalism is used to calculate the electron transmission in one-dimensional (1D) antidot arrays defined in a narrow two-dimensional electron-gas (2DEG) constriction. We show that when the modulation potential of antidots is weak the conductance bands can appear at the edges of the conductance plateaus of the narrow 2DEG constriction. In the case of strong modulation the calculated conductance of the 1D antidot arrays are seen to be characterized by two kinds of strong fluctuations, namely, the slow and rapid fluctuations, in high Fermi-energy range. The slow fluctuations result from wave interferences and the formation of the electron minigaps in the arrays and are insensitive to the temperature up to a few Kelvin, while the rapid fluctuations reflect the formation of the electron minibands and can be easily smoothed out by thermal averaging. Due to strong overlaps between the minibands associated with different 1D paths in the strongly modulated antidot arrays, the effects of the regular miniband formation may only be observed in the low Fermi-energy range, even at very low temperature.

Using an ultrahigh-vacuum scanning tunneling microscope, we have studied molecular-beam epitaxy-grown GaAs on vicinal (001)-(2×4) surfaces decorated at 450 °C with submonolayer coverages of InAs. On surfaces misoriented towards the [110] direction (A type), we have observed that the InAs nucleates in terrace defects, at the step edges, and on the terraces. Predominantly, we observe nucleation on the terraces with the formation of islands. The InAs islands are locally ordered into a (4×4) or a c(4×4) In-terminated reconstruction, which easily distinguishes them from the (2×4) GaAs substrate. The InAs growth is predominantly In-rich, although some islands exhibit domains of As-terminated (2×n) reconstruction. With increasing coverage, the anisotropy and density of the islands increases on the terraces while growth at the step edges remains almost constant. Furthermore, the size distribution of the islands in the [110] direction is narrow and independent of coverage for fractional coverages of InAs below θ=0.75 ML, indicating that there is a preferred island width. On surfaces misoriented towards the [1¯10] direction (B type), InAs nucleates predominantly at step edges and in the terrace vacancies. Island growth is observed on terraces larger than 8 nm, with the anisotropy of the islands dependent on the corresponding terrace width. As for the A-type surface, the islands are found to have a preferred width in the [110] direction of 4±1 nm. We suggest that strain effects are limiting the island size in the [110] direction.

Non-Ohmic and sample-size-dependent transport effects [i.e., Shubnikov–de Haas (SdH) and quantum Hall effect] of mesoscopic two-dimensional (2D) systems prove the occurrence of nonlocal contributions to the electronic conductance in these systems. However, this nonlocal regime accompanied by a non- equilibrium population of the edge states with respect to the 2D bulk state is quenched at rather low values of external electric fields or flowing currents, respectively. Beyond this quench, the bulk state is coupled to the edge by an increasing amount of electron transitions between the corresponding states. We analyze the non-Ohmic behavior of SdH oscillations at GaAs/${\mathrm{Ga}}_{\mathit{x}}$${\mathrm{Al}}_{1\mathrm{-}\mathit{x}}$As quantum Hall conductors on the basis of a model including edge and bulk transport. We deduce the current-dependent non- equilibrium population of edge and bulk states quantitatively. Further, we give estimates for the current ranges in which transitions of electrons between edge and bulk states due to elastic and inelastic scattering are relevant. The change of the typical nonequilibrium parameters as the equilibration length and the maximal difference of chemical potentials of edge and bulk states in tilted magnetic fields are also discussed.